New (improved!) fusor design

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Anze A Ursic
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New (improved!) fusor design

Post by Anze A Ursic »

Hello everyone,

For some time, I have been trying to achieve fusion with my cylindrical design. While I was able to ignite some beautiful Nitrogen and Deuterium plasmas, the current setup was just not capable of doing fusion due to several issues, including a complicated structure, feeding HV with wires to the system, an incredibly large chamber, outgassing, nonuniformity of the chamber etc... Over 100 tests and several revision of the fusor, we were unable to get the voltage past 12-13kV, because the plasma would start becoming unstable and just disappear. A quick glance at my profile will show the abundance of issues I have had throughout the years.

However, a new opportunity has just appeared as I was able to convince our lab owner to use another vacuum chamber system, which is smaller, holds pressure incredibly well (it was fully built by a professional with decades of experience), can go down to E-5 torr and is clean and ready to go. Because the rest of the fusor system (remote HV control, ballast resistor, Deuterium input regulation, video monitoring, lead shielding, etc...) is ready to go and just has to be transferred from the old vacuum system to the new, I have started designing a simple spherical fusor, with a grid that would sit atop of the HV feedthrough stalk.

Here is the catch; while I have previously fixed problems by trial and error, I am somewhat time constrained this time. I only have 7 or so weeks to do this, although it is something I can focus on 24/7, since I am here at university solely for this project now. Because of this time constraint, I figured I'd share my plan for the "new" fusor and see what people think. I believe I have included most, if not all, relevant information in the figures below, but please let me know if any additional info is needed. I'd be happy to provide it.

Please let me know of any potential pitfalls if you can spot them. Currently, I am mostly concerned about the tungsten wire diameter, which is 20 mil (0.5mm).

Any help is much appreciated!

Design:
design.jpg
Front:
front_view.jpg
Side:
side_view.jpg
Grid Design:
grid design.jpg
HV Feed:
hv_feed.jpg
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

So I just realized I could have gone into more detail on some aspects. But in general I'm just trying to build a spherical fusor of roughly 1.5" diameter that would sit atop a stalk coming directly from the HV feedthrough. Currently, a 0.093" steel rod carries the connection, but I will machine a longer and thicker stainless steel stalk on which I will attach the tungsten grid. I'll make sure the stalk is decently thicker than the tungsten grid wire so as to have a central plasma located in the very center of the device. The HV feedthrough would be connected on the other side to my Spellman DF3 -60kV power supply with a 100kOhm ballast resistor in series. 180 degrees from the HV feedthrough I would have a gas inlet for the Deuterium with a remote gas regulation system that's already built and tested. The "back" of the system would be connected to the pumps (mechanical and turbo). There is a langmuir probe on top of the chamber that reaches down towards where the grid would be but that is easily removable since I don't necessarily need it.
JoeBallantyne
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Re: New (improved!) fusor design

Post by JoeBallantyne »

Although I have not yet made neutrons with it, I certainly got very nice plasma, by using a 2mm stainless "drive shaft" stainless steel rod, a couple of 2.5mm drive shaft couplers, and a stainless wire shake mixer ball, all purchased from Amazon for a total of about $25 or so. All that was required to assemble was to cut the drive shaft to the appropriate length, and clamp things together with the couplers. Took maybe an hour or so max to put the cathode together. (Plus a couple of days for the orders to arrive from Amazon.)

Those shaker balls come in about 1" and 2" diameter sizes, and are made from about 1mm diameter stainless wire.

Pictures of the feedthrough/cathode setup are on this thread: viewtopic.php?p=96667

Joe.
Last edited by JoeBallantyne on Sun Sep 18, 2022 6:50 pm, edited 1 time in total.
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Richard Hull
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Re: New (improved!) fusor design

Post by Richard Hull »

Anze, I think you meant to say you were looking build a 1.3-inch spherical grid for your fusor.... Not to make a 1.3-inch spherical fusor.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: New (improved!) fusor design

Post by JoeBallantyne »

Links to the items on Amazon:

2mm stainless rod: https://www.amazon.com/gp/product/B00QBYPT6Y current cost $12

3mm brass drive shaft couplers: https://www.amazon.com/gp/product/B07VFFJBVH current cost $11

Stainless shaker balls: https://www.amazon.com/gp/product/B08NXDBFWD current cost $7

So for $30 plus tax you can make at least 4 cathodes of a couple of different diameters with very minimal effort.

Joe.
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

Richard - right, sorry, that was the size of the GRID, not the fusor. But either way, I hope that's an acceptable plan going forward. I know with just the new chamber system and simpler grid design, it will alleviate several issues.

Joe - Thank you! I am not sure about using those shaker things as cathodes during a fusion run, since my tungsten electrodes in my current setup glow red hot at ~13kV, but those couples are genious for making connections, thanks!

Anze
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Re: New (improved!) fusor design

Post by JoeBallantyne »

It all depends on how much power you are pushing through the cathode. To minimize sputtering (not sure, but I suspect brass is a lot more likely to sputter than stainless) you probably want to try to keep the current you push through the cathode to 20mA or less. The less the better as far as minimizing sputtering.

The shaker ball worked fine for me up to 31kV (the highest I could push it at the time) and up to 8 mA. My power supply was hard limited at about 8mA so I didn't go higher than that. The duration of the time you let it run also matters. Its all about the power dissipation. The heat builds up over time, and a vacuum is a real good insulator, so if you don't have some way to remove the heat, it can be a problem.

Joe.
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Re: New (improved!) fusor design

Post by JoeBallantyne »

The lower you keep the pressure in the chamber, the lower the current will be for any voltage applied. The higher the voltage, the more the fusion, so keep the pressure as low as you can and the voltage as high as you can with the plasma still lit, and that should minimize the current. Based on the graphs from a recent post by David Liam, doing so also optimizes the efficiency of the fusor.

Joe.
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

Joe, that's incredible. I would have thought at those power levels (31kV, 8mA) it would've melted without a doubt, but I guess I was wrong. I do still have a bunch of tungsten that's thin enough to be easily malleable (and already comes coiled so making rings is easy), so I'll try going with that, but that's an interesting idea going forward.

Anze
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Liam David
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Re: New (improved!) fusor design

Post by Liam David »

That looks like a much better chamber than your previous one. Bake it out if you can. If it was assembled correctly and you're careful with your modifications, you should be able to hit low e-7, high e-8 torr.

The total power that your cathode can support depends on several factors. The primary two are surface area (energy is lost mainly through radiation since conduction through your stalk is minimal) and of course the material. Use tungsten, molybdenum, or stainless--not copper, zinc, or any high vapor pressure metals or their alloys. Brass and bronze are a big no. Unless you have a high-current arc, all but the thinnest tungsten will survive 8 mA.

The primary reason that efficiency rises with voltage is that the cross-section rises quasi-exponentially. For a given power, more voltage will win every time (at least within the ranges we care about). Although my plots did show this effect to some extent, they would be more accurately interpreted thusly: since the neutron rate didn't increase as fast as the cross-section and sub-linearly with current, there were other mechanisms reducing efficiency.
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Re: New (improved!) fusor design

Post by Matt_Gibson »

I’m using .58mm tungsten wire and have pushed it up to around 700w for a few minutes at a time. It gets yellow hot, but hasn’t melted yet. I have a stainless mesh screen underneath (inside the flange) to catch anything in the event that I do melt down my cathode.

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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

Matt and Liam,

Thanks for the input. Much appreciated. Will continue going forward with the build and post some updates. Hoping to get "first light" by end of week.

Anze
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

New grid built! Very happy with how it turned out. SS Extender / stalk and 3 tungsten wires at the end forming a nice 1.5" diameter grid. Made sure to ground all the sharp edges. Now to clean it and put it inside the new chamber!
grid 1.jpg
grind 2.jpg
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Re: New (improved!) fusor design

Post by Matt_Gibson »

This ought to work much better for you.

-Matt
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Liam David
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Re: New (improved!) fusor design

Post by Liam David »

Tape! On a new copper gasket! It hurts my eyes to see!

You might want to remove the metal deposits on the inside ceramic.
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

Liam, I just realized the gasket doesn't go over the grid so I'm just happy there is a gasket there to begin with, otherwise I would've had to disassemble the stalk from the feed!
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Richard Hull
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Re: New (improved!) fusor design

Post by Richard Hull »

We live and we learn. Gaskets must be clean and pristine, untouched by human hands as with all internal surfaces under vacuum. Outgassing and virtual leaks are the result of any contamination. Tape, sticky residue is just one of many sources of contamination.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

I always wear gloves and clean every surface thoroughly with isopropyl alcohol and kimtech wipes. So hopefully it'll end up holding vacuum good!

Anze
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

I will probably ignite a quick air plasma with a 0-5kV power supply I have to test connections and make sure there is no arcing but should I then proceed with burning Deuterium plasmas or should I try and hit 20kV+ with something cheaper like Nitrogen to show it can handle such power levels?
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

Some update is in order. The new chamber outgassed pretty bad on the first day of pumping, monday. It has been under pressure since 2019 and at roughly 270-280 Torr, once I started pumping with the mechanical pump and reached around 5E-2 Torr, I sealed the chamber off and the outgassing was about 10 mTorr / minute. Then I just kept pumping until I was reaching 1.3E-2 Torr, which seems to be the lowest I can go with the mechanical. I just kept pumping and outgassing improved dramatically. Best so far was just 1.0E-2 in 15 min, so about 0.67 mTorr/min. Not bad considering the fact that this chamber is big, with a large number of connections and measuring devices attached and we haven't even turned the turbo on yet.

Anyway, I installed the old gas feed system on this chamber first, then replaced a 6" blank on one face of it since that blank had a large metal electrode sticking in the middle of the chamber. After each new component added / changed, I made sure to pump down all day next day to make sure I reach 1.3E-2 Torr in similar amounts of time and get the same outgassing. Today I installed my actual fusor and am currently pumping down to confirm I installed that right too.

I will try and ignite a quick plasma tomorrow if today's pumping tests go good and then proceed to clean the chamber by pumping for long periods of time with the turbo and see what pressures I can reach.

Thanks for the help everyone, will keep everyone posted... After 5 years of working on trying to get fusion, starting from scratch, I hope this time it actually works. I hope the combination of better chamber, simpler grid design and simpler construction (no ceramic beads and insulation) lets me push past that 20kV mark and get detectable fusion...
fusor_grid.jpg
Anze
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

Excellent news, today I ignited the first nitrogen plasma to test for shorts, stability, etc. and was able to reach 16.6kV applied voltage at roughly 2-4mA for over 8 minutes before I shut it down. The plasma was incredibly stable, not a single flicker occurred the entire time. Anyone who has followed my previous design knew I struggled with plasma stabilities, with most of my plasmas struggling to reach 12-13kV and extinguishing after 10-30 seconds. I will be performing further tests tomorrow before switching to Deuterium and pushing past the 20kV mark on the way to fusion.

Anze
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Re: New (improved!) fusor design

Post by Matt_Gibson »

Looks good. I’d do some extended “heating” runs to make sure things are clean before pushing voltage higher. Work your way up.

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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

Matt,

will do tomorrow. Today I was tired and didn't want to push past 17kV due to X-ray concerns, but actually, we are remote controlling and monitoring this from another room and we have lead shielding so we should be good. I'm just a bit limited by what my school safety folks say.

Anze
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

Today, still running Nitrogen, we were able to hit 22.0kV. Quite good! I was ramping up voltage slowly, but it's a breeze with this new chamber. Not a single instability or issue to be had.

Anze
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Anze A Ursic
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Re: New (improved!) fusor design

Post by Anze A Ursic »

Talk about thin ice... safety officers came to the fusor lab today looking to record X-ray exposure and at 22kV (still nitrogen), max dose rate was 1.93mR/hr close to the viewport. Per law, if it were 2.00mR/hr or more, we would have been under much greater supervision and this would have probably been delayed further. Apparently 2.00mR/hr is the maximum legal limit for a publicly-accessible area, which our lab technically is.

Anyway, we are now good for a fusion run, they will bring neutron counters next time. Unofficial date is Friday 10/7/2022, so hopefully this beauty works well just one more time!

Anze
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